Thrombospondin (TSP) is an extracellular matrix component with multiple biological functions. Several reports have indicated an anti-angiogenic activity for TSP. In preliminary studies, we have noted decreased thbs-1 mRNA expression in highly metastatic melanoma, lung and breast carcinoma cells, consistent with a suppressive activity. In order to confirm the function of TSP, a thbs-1 expression construct was transfected into the metastatic MDA-MB-435 breast carcinoma cell line. Injection of thbs-1 and control transfected lines into nude mice indicated that TSP expression results in a significant decrease in primary mammary tumor size as well as incidence of distant metastatic disease. In vitro analysis to date indicate no difference in the anchorage-dependent or - independent growth of control and thbs-1 transfected cells. Preliminary data using Factor VIII immunostaining, however, indicate a significant reduction in capillary density in primary tumors produced by thbs-1 transfectants, indicative of an angiogenesis-suppressive effect. Should additional studies confirm an antiangiogenic effect of TSP, characterization of the functional domains of the TSP protein will be conducted and tested in mice.